That's just what people like to say without any quantifiable metrics (usually just with some random data points about this or that bug, but not quantifiably compared to previous releases).
Millions of people use it everyday for professional work, including in film and music studios, where stability and timely performance are much more paramount than for some corporate cubicle running Excel.
I've not seen anything actually actively deteriorate. The Finder is much better, Spotlight is much more robust than before, apps run fine, etc.
There have been bad moves, like the change to their custom DNS responder thing which was not ready for production. Or some glitches here and there. But I've seen such things all the way down to 10.1 were I started. There will always be bugs when you add new features (and even when you don't add anything, as technologies around your OS also change).
Plus the upcoming release is a "Snow Leopard" like bugfix affair too, which will help things.
This can be said about Windows
The main issue with Windows, if there is one nowadays, is some UI cohenrece, not bugs and stability.
It is ? You are referring to Yosemite ? Genuinely curious...
* Trying to save files in Pages/Numbers, I get an error "Cannot save file. File does not exist." (Well of course it doesn't exist, I haven't saved it yet!) Lost several hours of work multiple times because I couldn't save files. And that is a total shit error message.
* Trying to move folders on a remote server. I get a dialog that says "Finder wants to make changes" and asks for my admin password. (What changes? WTF?) Files got deleted when moved. And also a total shit 'warning' message.
OS X is far more frustrating and useless than any other OS I've ever used. It doesn't give reasonable error messages, and it doesn't keep your stuff safe. It's a confusing minefield.
As long as everything I do is in a non-Apple application, I can get things done. And don't even get me started on OS X mouse support.
All software sucks. The only reason you think OSX sucks more than other OSs is because you haven't had time to discover the work-arounds for annoying buggy behaviour.
Extrapolating from that experience (which might be entirely to the remote server setup or network issues) is not exactly scientific.
The only way to fix it AFAIK is to reboot. (Doesn't that remind you of Windows?)
My colleagues have encountered the same issue.
Individual products will always have their pros/cons, and the company will have strengths (design) and weaknessess (social, some cloud services).
But to say they're bad at software just isn't true.
